1. Field of the Invention
This invention relates to a hydraulic brake valve system for use in a hydraulic circuit including at least one hydraulic motor for the actuation of an inertia body such as, for example, a starting operation, a braking operation, etc., of a construction vehicle.
The brake valve system of the kind specified which has so far been employed comprises a counterbalance valve installed in a hydraulic fluid circuit connecting a hydraulic motor with a manually operated directional control valve connected to a pressurized fluid supply source, and a cross-over safety valve comprised of two relief valves installed in parallel between the counterbalance valve and the hydraulic motor, the arrangement being made such that, when the manually operated directional control valve is manipulated to change over the counterbalance valve, the pressurized fluid from the pressurized fluid supply source will circulate in either direction from a driving (or normal running) fluid line to a braking (or reverse running) fluid line of the hydraulic fluid circuit or from the braking line to the driving one thereby driving or braking the inertia body connected to the hydraulic motor. Both the magnitude of acceleration of the inertia body at the time when starting and the magnitude of the braking force to be applied to the inertia body when the manually operated directional control valve is manipulated so as to change over the counterbalance valve depend on the set pressure for the above-mentioned two relief valves constituting a cross-over safety valve. The set pressure will increase as the weight of the inertia body increases. For this reason, the hydraulic fluid circuit employing the conventional brake valve system has been disadvantageous in that when starting, ending acceleration of, commencing deceleration of, stopping, etc., a large inertia body like a construction vehicle such as, for example, a hydraulic shovel, a hydraulic crane, etc., the fluid pressure discharged by the hydraulic pump will increase suddenly by the inertia of the inertia body and a high acceleration or deceleration energy is transmitted to the hydraulic motor thus causing an abnormally large shock in a machine having a large mechanical backlash such as, for example, gears, etc. In obsevation of the waveform of the fluid pressure in the circuit, at the time when starting the inertia body, a sharply rising peak pressure will occur in the driving fluid line of the hydraulic fluid circuit, whilst in case of ending acceleration of the inertia body and when commencing deceleration thereof or stopping it, a sharply rising peak pressure will occur in the braking fluid line of the circuit. It is envisaged that the generation of such sharply rising peak pressures forms one of the causes of the shock. The generation of such a peak pressure is due to the fact that the cross-over safety valve which have so far been employed in the hydraulic fluid circuit of this kind can give such an operating function as providing a set pressure rising at only one stage so that the fluid pressure is varied quickly from a low pressure to a high pressure. To cope with this, it is envisaged to increase the set pressure in two stages. To achieve the purpose, however, it is necessary in the conventional arrangement to vary the fitting length of the spring for regulating the set pressure and to provide a movable part thus making the construction of the arrangement complicated and increasing the cost thereof.
One embodiment of the hydraulic brake valve system wherein the set pressure can be increased in two stages is disclosed in the Laid-open Publication No. Sho 56-103359 of Japanese Utility Model application published on Aug. 13, 1981 in Japan. According to this prior art, the conventional crossover safety valve is replaced with two relief valves 16 and 17 mounted in hydraulic conduits 8 and 9, respectively, to which branch conduits respectively branched from the conduits 9 and 8 are connected as their respective primary passages 14 and 15. These relief valves 16 and 17 form a hydraulic brake valve system in association with a counterbalancing valve 5. Each of the relief valves 16, 17 is constructed such that a poppet 19 located oppositely to the primary passages 14 or 15 is urged by a spring 20 to a valve seat 18. The poppet 19 is formed with a rod chamber 22 in which is inserted one end of a rod 21 having a sectional area S.sub.2 smaller than the sectional area S.sub.1 of the seat and a restricted passage 23 defined to commuicate the rod chamber 22 with the primary passages. The other end of the rod 21 is inserted into a spring chamber 24 and is urged against the poppet 19 by the force of a spring 26 interposed between the rod 21 and a spring retaining surface 25 of the spring chamber 24. In addition, the rod 21 has a small protrusion 27 formed at the other end thereof and which serves to set the maximum amount of movement of the rod in co-operation with the spring retaining surface 25. The spring 26 has, of course, a spring constant smaller than that of the spring 20. Therefore, the spring 26 is relatively weak in its biasing force. Obviously, in this prior art, a movable part such as, for example, the rod 21 is located in the relief valve in addition to the poppet. To set a certain set pressure, it is required to adjust the biasing forces of the two springs 20 and 26. Therefore, the construction of the brake valve system becomes complicated thereby increasing the cost thereof. Further, in the above-mentioned prior art embodiment, the stroke of the rod 21 is set to be comparatively long, and so the response thereof to a sudden variation in the fluid pressure is not rapid. Further, because the set pressure is set in two stages in each of the relief valves, the stablity of valve functions at the time when the valves are changed-over is unsatisfactory.